3 Introduction Adaptive transmission techniquesvarying the transmission power, packet length, coding rate/scheme, and modulation technology over the time-varying channelWith the new high-speed IEEE a MAC/PHY,we can transmit packets with different data rates ranging from 6 to 54 Mbps  a Multi-rate, Multi-range IWLAN (MMI-WLAN)However, all mobile hosts (MHs) can not insist on using the highest-level modulation schemethe data rate is inversely proportional to the transmission distance between a pair of MHs

5 Related Work: Auto Rate Fallback (ARF)The sender selects the best rate based on information from previous data packet transmission, incrementally increasing or decreasing the rate after a number of consecutive successes or lossesThe rate selection is performed by the sender, the channel quality estimation is also performed by the senderARF provides a performance gain over pure single-rate IEEE

6 Related Work: Receiver-Based Auto Rate (RBAR)The sender Src chooses a data rate based on some heuristic, and then stores the rate and the size of the data packet into the RTSNode A, overhearing the RTS, calculates the duration of the reservation Drts.The receiver Dst uses information about the channel conditions to generate an estimationDst then selects the rate based on that estimate, and transmit it and the packet size in the CTS back to the senderThe Reservation SubHeader (RSH) in MAC header of the data packet is used to update the NAV to finally confirm the reservationIn RBAR, the rate selection and channel quality estimation are located on the receiverRBAR yields significant throughput gains as compared to ARF

7 Related Work: Opportunistic Auto Rate (OAR)The key OAR mechanism is for a flow to keep the channel for an extend number of packets (instead of for a single packet) once the channel is able to transmit at rates higher than the base rateOAR uses the fragmentation mechanism in IEEE as the basic transmission schemeOAR receivers continually monitor the channel quality, and if a significant change is detected, additional RSH messages are used to adapt the rate

16 Design ObjectsThe first aim is to keep the channel for an extended number of packets once the channel is measured to be of sufficient quality to allow transmission at rates higher than the base rate.The second aim is for nodes near the fringe of AP's transmission range to transmit packets at a higher rate.

20 Transmission with Best Modulation SchemeAP can uses, for example, broadcasting approach to let all MHs know their locations and use the best modulation schemeIn addition, each MH can overhear packets transmitted in air, measure their signal to noise ratios (SNRs), estimate the distances and the modulation schemes to be adopted between itself and the transmitting nodes, and finally record these information in a so-called neighbor-list.AP can then obtain the information about the MHs in its transmission range, and collect these MHs' neighborhood information by means of the neighbor-list delivered with some kinds of periodical reports or routing information exchanges.With these, AP can decide the relay node for each MH without the requirement of exact knowledge about the directions of these MHs.

21 Throughput of IEEE 802.11 MAC and RAAR in Single Node EnvironmentIEEE a PHY Models

27 IEEE 802.11 Anomaly The overall transmission timeThe time for the jth transmission attempt in IEEE

28 IEEE AnomalyAssume Ptx and n are fixed, and s and m are given according to the locations, the overall transmission time in region j can be defined asBecause the long term fairness of CSMA/CA, the utilization factor in region j can be obtained by

29 IEEE 802.11 Anomaly Throughput AnomalyIn above no particular terms with regard to Rj are involved in the final formula, which implies that a node located in an inner region and using a higher rate to transmit l-bytes data obtains the same throughput as a node located in an outer region and using a lower rate for the same data.Contrary to all expectations of the multiple-rate PHY, this result exhibits the fact that a higher data rate does not bring the throughput higher than the others if it competes with a lower rate.

30 Solving the Performance Anomaly Problem of IEEE 802Solving the Performance Anomaly Problem of IEEE MAC with Degraded RAARAn example of the control flow of D-RAAR

31 Throughput of D-RAARIf the value of the denominator is similar to that of IEEE MAC, in the numerator in fact represents the gain factor that D-RAAR can increase on the throughput

38 ConclusionIn Relay-Based Adaptive Auto Rate protocol, a relay node is added to increase the system throughputRAAR will revert to a degraded version, namely D-RAAR, and perform the normal IEEE fragmentation with variable data rates suggested by AP, when no relay node can be found or the relay node is missed due to mobilityAnalysis and simulation both indicate that with this scheme, significant throughput improvement can be achieved for nodes located at the fringe of the AP's transmission range, thus significantly improving the overall system performanceExperimental results show that, based on the measured data in indoor environment, adopting RAAR in the outermost four regions while using D-RAAR in the innermost four regions can effectively combine both methods' benefits and remarkably improve the system throughput